Such screw stoppers are widespread in cans available on the market. Occasional complaints regarding escaping fuel indicate that in these cases the screw stopper was not correctly operated, and the seal was not able to carry out its function. Thus, for example, dirt in the threaded region can make the screwing on of the screw cap so difficult that the closure position is thereby simulated. Likewise, a closure position is simulated if the screw cap is screwed in a tilted position on to the pouring spout, in which case naturally the threading is partially damaged. Another fault consists in that the screw cap is screwed on with excessive force and then either the threading is so damaged that the screw cap can no longer be held correctly, or the seal is irreversibly deformed so that it can no longer carry out its function. The latter is to be expected predominantly in the case of seals of synthetic plastics material. Naturally, both kinds of damage can occur together.